This invention relates to cutting a web of moving sheet material and more particularly to the cutting of a traveling continuous web into a plurality of separate sheets.
It is known to cut a longitudinally traveling continous web of paper or the like into separate sheets by means of a rotating cylinder extending transversely of the path of web travel and having a single axially extending knife thereon for cutting engagement with the web. Two such rotating cylinders, arranged on opposite sides of the web, have usually been utilized, with the oppositely rotating knives cooperating to provide the cut.
Heretofore, each rotating cylinder was often constructed either as a solid piece of metal or alternately as a thick-walled metal tube having a hollow axial bore and unsupported inner wall.
It has been desirable to reduce the forces of inertia involved especially with variable speed cutting cylinder drives. The above-identified U.S. patents disclose devices which will reduce the inertia. Such devices contemplate providing a rotatable cutting cylinder with a thin wall and which is disposed concentrically over a fixed solid cylindrical shaft-like core of heavy construction. In U.S. Pat. No. 3,683,734, a plurality of annular bearings are disposed between the cylinder and core and not only asist in cylinder rotation but also support the thin cylinder wall against distortion due to radial cutting forces. Other support concepts are disclosed in these patents as well.
It has been observed that, when mechanical supports are provided between the cylinders and solid cores of a dual cylinder arrangement, an unusual phenomenon occurs during high speed web cutting, such as that which requires a rotative speed of the cutting cylinder of from 150 to 300 rpm. The phenomenon comprises a high frequency ringing, such as a bell-like or piano string sound. Such ringing is accompanied by incomplete or ragged cuts of the traveling web. It has been found that increasing the overlap of the cutting knives will help overcome the poor cuts, but the knives then are subject to faster wear and the ringing has not been eliminated.
At relatively low rmp of the cutting cylinders, the ringing and incomplete cuts do not occur. Instead, they commence abruptly as the drive speed reaches a certain level.
It is a task of the present invention to find a solution to the aforementioned undesirable ringing and poor cuts, while at the same time permitting the knives to maintain their normal overlap and maintaining low inertia of the cutting cylinders.
The present invention is based on the discovery of the causes of the problem, and the development of a solution therefor.
In considering the problem, the inventor came to the conclusion that as the opposed knives came together for a single cut of the web, an inward radial force was created not only on the thin cutting cylinder, but also through the intermediate bearings to the solid inner support shaft. This caused minute short-term distortions of not only the cylinder, but also of the support shaft. As successive cuts are made, the shaft is subject to a plurality of distortion inducing forces. If this plurality of successive forces occurs at a rate close to the assembly's resonant frequency, the magnitude of the deflections increase.
As to the undesirable ringing which was believed to emanate from the solid support shaft, the inventor studied the concept of natural resonance of the cutting cylinder, the solid support shaft and the assembly of both. The resonant frequency of a body subject to vibrations is proportional to the mass of the body in accordance with the formula: EQU rf.about.1/.sqroot.M
where rf is the resonant frequency and m is the mass.
The mass of the thin-walled cutting cylinder is a relatively low and thus it has a relatively high resonant frequency above the cutting frequency. The mass of the solid supporting shaft is relatively high, resulting in a substantially lower natural frequency for the assembly cutting cylinder and support shaft, a frequency which is close to the frequency of distortions caused by successive cuts. This was determined to be the cause of the ringing sound during fast cutting.
At low rpm of the cutting cylinders, each single distortion from a single cut was of low amplitude and was damped out sufficiently by natural losses during rotation before the knives came together for the next cut. However, as the rpm increased, the frequency of the distortions increased to the point where a combination of radial and torsional oscillations occurred in the shaft, resulting in turbulant vibrations in the shaft. These vibrations were not damped out sufficiently between high frequency cuts. This was believed to be not only the cause of the ringing but also the cause of the cutting knives not coming together properly for a clean cut of the web.
With this discovey of the causes of the ringing sound and ragged cuts during high speed operation, the inventor has developed a solution to the problem.
Broadly, in accordance with the various aspects of the invention, it was determined that the ring could be minimized by increasing the natural resonant frequency of the assembly of the cylinder and supporting shaft to above the cutting frequency by substantially decreasing the mass of the support shaft. In addition, it was determined that the ragged cuts could be substantially eliminated by damping the amplitude of vibrations of the support shaft during high speed cuts so that the vibrations caused by each successive cut are isolated and do not compound each other in the shaft to change the knife alignment.
More specifically, the support shaft is formed by a body having an imperforate cylindrical wall approximating the thickness of the thin wall of the cutting cylinder. The cylindrical wall of the support shaft is closed at both ends to form an uninterrupted cylindrical cavity extending from end to end of the cylindrical wall. In addition, the cavity of the support shaft is filled with a flowable vibration damping material which is not firmly affixed to the cavity walls and thus forms no significant part of the mass of the support shaft.